The present invention relates generally to on-line measurement of properties of sheet material as the sheet material is being manufactured and, more particularly, to methods and apparatus for measuring mechanical properties of webs of sheet material as the webs are being manufactured by measuring the propagation speed of ultrasonic energy through the webs. While the present invention-can be applied to webs of a variety of materials, it is particularly applicable and has been initially developed for use with webs of paper and, accordingly, will be described with reference to this application.
Measurement of the speed or velocity of propagation of ultrasonic energy in paper sheets is well known as a nondestructive test for predicting the mechanical strength of paper sheets and other structures Constructed from paper sheets, see, for example, U.S. Pat. No. 4,574,634 which is incorporated herein by reference. Many manufacturers have replaced less sophisticated destructive tests with such nondestructive tests and routinely perform the nondestructive ultrasonic energy tests on samples of finished paper products. Unfortunately, as long as the ultrasonic tests are performed off-line on finished paper products, substantial amounts of defective product may be produced before it has even been determined that the product is defective.
To correct this problem, on-line testing using ultrasonic energy has been pursued. See, for example, U.S. Pat. Nos. 4,291,577 and 4,730,492 which are incorporated herein by reference. These on-line testing arrangements utilize rotating wheels or drums having surface mounted transducers which are rotated with a web of paper such that there is effectively no relative motion between the transducers and the web being tested. Two or three transducers are oriented on the drums or wheels to make substantially simultaneous contact with the web of paper.
An impulse of ultrasonic energy is generated at a transmitting transducer when the transducers are in contact with the web. The impulse is then detected at a second receiving transducer or second and third receiving transducers. The velocity of the ultrasonic energy through the web is then calculated using the arrival time or arrival time difference if three transducers are used) and the distance between the transducers. Three transducers were provided in an attempt to remove timing errors introduced into the calculated velocities by phase shifts in the electrical and mechanical interactions of the transducers.
Unfortunately, none of the ultrasonic energy on-line testing arrangements pursued to date has resulted in a practical commercial measurement for webs of paper or other materials as they are being manufactured. Accordingly, a need remains for an accurate and reliable testing arrangement to determine mechanical properties of webs of sheet material as the webs are being manufactured by measuring the propagation speed of ultrasonic energy through the webs. Preferably such an arrangement would tolerate relative motion between ultrasonic transducers and webs of material being measured for association with conventional web scanning equipment.